To do this little trick, you need to know two things...how much health you have in normal form, and how much you have in bear. as a 67 druid, I was killing some guys in bear form and I lost about 2000 health. I then popped out of bear to heal and then I realized I had full health...when I shifted back to bear, I still had full health...this works because I had 9k health as a bear, and 7k health as a nelf. having a bit more then your nelf health in bear form means that you can shift out and then shift back in and you'll get full health in bear because you had full health in nelf.
If I wasn't as clear as could be, just say so